refactor: restructure repo into core/app modules with per-study folders

Reorganize Palladium codebase into a modular architecture with `core/`
shared logic and `app/` Streamlit UI, separating per-study assets into
`studies/YYYYMM_<Vendor>/` folders containing notebooks, seed data, and
configuration. Update README to reflect new structure, add `.gitignore`
entries for `.env` and study exports, and refresh component documentation.

core/calculations/payback.py

@@ -0,0 +1,63 @@
+"""
+Payback period.
+
+Linear interpolation within the crossing year: returns the moment when the
+running net cashflow first turns non-negative.
+
+Inputs are *risk-adjusted, undiscounted* cashflows by convention (TEI shows
+"<6 months" payback for the Amazon Connect composite using the risk-adjusted
+nominal cashflows from the Cash Flow Analysis table).
+"""
+
+from __future__ import annotations
+
+from collections.abc import Iterable
+
+
+def payback_years(
+    initial_cost: float,
+    yearly_net_benefits: Iterable[float],
+) -> float | None:
+    """
+    Years until cumulative net benefits cover the initial cost.
+
+    Args:
+        initial_cost: positive number — undiscounted year-0 outlay.
+        yearly_net_benefits: sequence of (benefits − costs) per year, Y1+.
+
+    Returns:
+        Float number of years, or ``None`` if payback is never reached
+        within the supplied horizon.
+
+    Example::
+
+        >>> # Amazon Connect: initial cost $1.196M, Y1 net $19.998M
+        >>> # → ~0.06 years ≈ 0.7 months — well under 6 months.
+        >>> round(payback_years(1_196_250, [19_997_953, 31_562_489, 47_443_905]), 3)
+        0.06
+    """
+    remaining = float(initial_cost)
+    if remaining <= 0:
+        return 0.0
+    cumulative_year = 0
+    for cf in yearly_net_benefits:
+        cumulative_year += 1
+        cf = float(cf)
+        if cf <= 0:
+            remaining += -cf  # net loss this year increases the gap
+            continue
+        if cf >= remaining:
+            # Crossing happens partway through this year.
+            fraction = remaining / cf
+            return (cumulative_year - 1) + fraction
+        remaining -= cf
+    return None
+
+
+def payback_months(
+    initial_cost: float,
+    yearly_net_benefits: Iterable[float],
+) -> float | None:
+    """Same as :func:`payback_years`, expressed in months."""
+    yrs = payback_years(initial_cost, yearly_net_benefits)
+    return yrs * 12.0 if yrs is not None else None